1. Field of Invention
This invention relates to systems for handling of bulk liquid cargos in tankvessels in emergencies when cargo tanks have been flooded or when the normal cargo system has been disrupted by a casualty.
2. Description of the Prior Art
Most state of the art bulk liquid cargo carrying tank-vessels are provided with cargo handling systems which include piping connecting loading midship manifolds and deck headers at one end through high capacity bottom-located cargo pumps, cargo mains and branches to "bellmouth" intakes fixed close to the bottom of each cargo tank at the other end.
Cargo is generally loaded in these vessels through the midship manifolds and deck headers to "drop lines" extending vertically from the deck headers to the bottom mains and from there, by proper alignment of valves, to the cargo tanks through branches and suction bellmouths in the bottoms of the respective tanks. In some vessels final distribution to the tanks from the drop lines to the tanks is made wholly or in part by means of sluice valves in tank bulkheads.
During normal loading, cargo is pumped "over the rail" into the deck manifolds by shoreside terminal pumps and, with the drop line valves properly aligned, flows under the influence of gravity and the shoreside pumps to charge the bottom mains for distribution directly to the cargo tanks, bypassing the vessel's cargo pumps and pumproom in the process.
During normal cargo discharge, by properly aligning the valves, one or more bellmouth intakes can be connected through the branches and bottom mains to the cargo pump system to produce suction in selected tanks for removal of cargo from those tanks to the deck headers for transfer to other tanks or ashore. The bellmouth intakes are located near the bottom of the tank and generally near the aft bulkhead thereof so that the maximum amount of cargo may be removed from the tank through high-capacity centrifugal cargo pump and large-diameter cargo piping systems. With this arrangement, by trimming the vessel by her stern, the bellmouth intake will remain immersed and capable of withdrawing cargo for the maximum length of time. Cargo remaining in the tank after this is removed by a lower capacity, positive displacement cargo stripping pumps through a smaller diameter pipe, parallel stripping system.
When a loaded tankvessel is collided with or goes aground with a hull rupture in way of her cargo tanks, seawater floods into a ruptured tank displacing the cargo in the tank as a function of the level of the highest point of damage in the hull. Where the cargo is lighter than the seawater, as the majority of liquid cargoes are, the displaced cargo, discharged to the sea as a spill, flows out rapidly at first and then gradually decreases in flow rate as the water/cargo interface approaches a natural equilibrium based on relative head between the cargo and sea. This natural equilibrium or "water bottom" is unstable, easily upset by relative motion between the vessel and the sea. Continuing spillage will normally result from wave and current action or from further movement or motion of the ship.
Another problem often encountered when a tankvessel has a casualty is that the bottom mains and/or branches of the cargo suction system are frequently disrupted by the upset or penetration of the hull so that communication between the suction intakes in the tanks and the cargo pumps in the pumproom is interrupted and normal cargo removal precluded.
If the cargo system were to be adapted to function in each tank, regardless of the flooded condition of that tank or the condition of the cargo suction system, the size of the spill could be significantly reduced since cargo could be transferred from the holed tank(s) before equilibrium is reached. It would also be of great help if the cargo in the holed tank(s) could be transferred or reduced to a degree that the resulting natural equilibrium is stabilized to prevent further spillage from the vessel caused by relative motion between the vessel and the sea. Such stabilization would also allow more radical movement of the vessel to speed her salvage and removal from further peril.
Because of the configuration of present cargo handling systems, however, the water bottom in a flooded tank will envelope the bellmouth suction intakes and/or disruption of the bottom cargo mains will preclude communication with the cargo pumps, effectively disabling the vessel's cargo pumping system in the flooded tank and/or tanks beyond the disruption. Stranded tankers have therefore been required to await the arrival of emergency "over the top" pumping equipment to have cargo in their holed tanks removed or reduced. This has resulted in the loss of all of the cargo whose discharge was required to achieve a natural interface and has involved a substantial delay in salving a casualty, prolonging the spillage of cargo and endangering the vessel and remaining cargo by delaying salvage efforts until natural equilibrium in the holed or disabled tanks is stabilized. In the EXXON VALDEZ grounding in Alaska in 1989 for example, several days passed before sufficient emergency "over the top" pumping capacity arrived and could be rigged to stabilize the several water bottoms created in that vessel by the grounding, substantially prolonging the duration (and increasing the ultimate quantity) of spillage from the vessel and the length of time she remained on her strand and vulnerable to the elements.
A device intended to accomplish similar objectives to those of the present invention is disclosed in U.S. Pat. No. 4,389,959. This device proposes a system to offload cargo from a holed vessel tank by means of an additional, independent cargo handling system fixed at a designated distance above the hull bottom and the vessel's indigenous cargo handling system. Aside from the added expense, weight and increased complexity of a system requiring an additional cargo handling system with redundant piping, cargo pump and valving, this system also introduces increased maintenance effort and increased probabilities of failure inherent in a system that would normally be used only in an emergency.
In U.S. Pat. No. 5,095,836, dated Mar. 17, 1992, emergency bulk liquid handling systems for vessels are disclosed which also address the same problem as does the present invention. These prior systems, however, either utilize the bottom mains of existing cargo systems or require the utilization of a deck-mounted main system to accomplish their purposes. Since the system would be disabled by disruption of the bottom mains in the former embodiment or would require an entirely different cargo distribution and collection system in the latter, the present invention is readily distinguishable over the systems disclosed in the above-mentioned patent.